This invention relates to ultrasonic apparatus and more particularly, to methods and apparatus for detecting leaks and malfunctions of mechanical parts by ultrasonic means.
It is well known that ultrasonic generators and detectors can be used to locate leaks, e.g. in pipes. Such a system is shown in U.S. Pat. No. 3,978,915 to Harris. In that arrangement ultrasonic generators are positioned in a chamber through which the pipes pass. At the ends of these pipes, exterior to the chamber, ultrasonic detectors are located. At the point where a leak occurs in the pipe or the pipe wall is thin, the ultrasonic energy will enter the pipe and travel along the pipe to the end where the detector resides. Thus the detector will receive a signal indicating the existance of this leak or weak spot. Since ultrasonic energy used for such purposes is generally in the range of 40 KHz, it is too high in frequency to be heard by a human being. Thus, means are provided for heterodyning or frequency shifting the detected signal into the audio range and various schemes are available for doing this.
By locating an ultrasonic generator in a closed chamber, a standing wave pattern with peaks and nodes, is established. If a node occurs at the position of a leak or weak spot, no ultrasonic energy will escape and the defect will not be detected.
In certain instances, e.g. in detecting the malfunction of bearings, an ultrasonic detector is mechanically coupled to the casing of the bearings so that the vibrations caused by the malfunction can be mechanically transmitted to it. With such an arrangement the frequency is not set by an ultrasonic generator, but is created by the mechanical vibration itself. In such a case the ultrasonic detector circuit must be capable of sweeping over a band of frequencies to locate the one that is characteristic of the malfunction. This is usually accomplished by a heterodyning circuit which can be tuned to various frequencies, much in the manner of a radio receiver.
Ultrasonic transducers generally produce a low voltage output in response to received ultrasonic energy. Means have been proposed for increasing this output. For example, in U.S. Pat. No. 3,374,663 to Morris it is suggested that an increase in the voltage output can be achieved by serially arranging two transducers. It has been found, however, that with such an arrangement a typical transistor preamplifier loads the transducers to such an extent that the gains achieved by stacking them serially are lost. The Morris patent proposes the use of a triple Darlington configuration in order to produce a sufficiently high input impedance to prevent this degradation in the signal produced by the stack of transducers. Unfortunately, the transducers in this arrangement are not placed so that they both readily receive ultrasonic energy. Thus the Morris arrangement is not entirely satisfactory.